1. Field of the Invention
The present invention relates to an apparatus and a method for encoding and decoding synthetic images, and more particularly, to an apparatus and a method for encoding and decoding a coordinate interpolator representing the location of each vertex of an object in a keyframe-based graphic animation using coordinates of the vertices each consisting of x, y, and z components.
2. Description of the Related Art
Three-dimensional (3D) animation techniques have been widely adopted in 3D computer games or virtual reality computer applications. Virtual reality modeling language (VRML) is a representative example of such 3D animation techniques.
International multimedia standards, such as MPEG-4 binary format for scene (BIFS) and virtual reality modeling language (VRML), support a keyframe-based 3D animation using an interpolator node. In MPEG-4 BIFS and VRML, there are various kinds of interpolators including a scalar interpolator, a position interpolator, a coordinate interpolator, a orientation interpolator, a normal line interpolator, and a color interpolator, and these interpolators and their functions and characteristics are shown in Table 1.
TABLE 1InterpolatorCharacteristicsFunctionScalarLinear interpolation ofCapable of representinginterpolatorscalar variationsarea, diameter, andintensityPositionLinear interpolation on 3DParallel movement in 3DinterpolatorcoordinatesspaceOrientationLinear interpolation of 3DRotation in 3D spaceinterpolatorcoordinate axis andamount of rotationCoordinateLinear interpolation of3D morphinginterpolatorvariations in 3DcoordinatesNormal lineLinear interpolation of 3DCapable of representinginterpolatorcoordinates of normal linevariations in 3D vector ofnormal lineColor interpolatorLinear interpolation ofCapable of representingcolor informationvariations in color
Among the interpolators shown in Table 1, the coordinate interpolator is used to represent information on the location of each vertex constituting a 3D object in a keyframe-based animation and are comprised of keys and key value fields. Key fields represent the position of each keyframe on a temporal axis using discontinuous numbers in a range between −∞ and ∞. Each of the Key value fields specifies information on the position of each of the vertices constituting a 3D object at a certain moment represented by each of the keys and consists of three components x, y, and z. Each of the key value fields includes as many key values as each of the key fields has. In such a keyframe-based animation, predetermined keyframes are located at arbitrary places on a temporal axis, and animation data between the keyframes are filled by linear interpolation.
Since linear interpolation is employed in MPEG-4 BIFS and VRML, a considerable amount of key data and key value data is required to represent an animation as naturally and smoothly as possible using linear interpolators. In addition, in order to store and transmit such a natural and smooth animation, a storage of a large capacity and a considerable amount of time are needed. Therefore, it is preferable to compress interpolators in order to make it easier to store and transmit the interpolators.
In predictive MF field coding (PMFC), which is one of the methods for encoding and decoding interpolator nodes that have been adopted in MPEG-4 BIFS, key value data of a coordinate interpolator are encoded using a quantizer, a differential pulse code modulation (DPCM) operator, and an entropy encoder, as shown in FIG. 1. Referring to FIG. 1, the quantizer and the DPCM operator removes redundancy of the key value data, and the DPCM operator outputs the results of its operation to the entropy encoder. However, PMFC is not sufficiently effective in encoding key value data because it entropy-encodes only differential data obtained from a general DPCM operation and considers only a spatial correlation between vertices constituting a 3D object in an animation without taking into consideration a temporal correlation between such vertices, which is very important in a keyframe-based animation.